Target domain profiling of target optical surfaces using excimer laser photoablation

ABSTRACT

A method for inducing cylinder power into an optical surface which comprises the steps of providing a target optical surface to an apparatus capable of indexing the position of said target to the beam path of a laser capable of photoablating the material of said target, passing said target through the domain of said laser along at least one axis and controlling the product of the intensity of said laser with time in order to control the amount of ablation of said target along at least one axis of said target.

BACKGROUND OF THE INVENTION

A number of methods are known for shaping optical surfaces. Perhaps, theoldest known method is the use of a lathe to reconfigure the surface ofan optical article. This method, of course, dates back to the firstlenses and the method is used even to the present.

Methods have also been developed for casting or molding opticalsurfaces. Even these methods, however, depend upon lathing techniques togenerate the mold pieces being used to mold the finished opticalarticle. More recently, the idea of using a high energy laser toselectively ablate the surface of an optical article has been put forth.

The present invention allows the use of an excimer laser to selectivelyalter the surface of an optical article and provides a highly effectiveand precise means for doing so.

SUMMARY OF THE INVENTION

The present invention involves a method for reconfiguring the surface ofan optical article. The present invention is particularly useful inproducing toric surfaced optical articles with much more accuracy andprecision than have been heretofore available.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is the graphical representation of the shape of the cross-sectionof an excimer laser beam and shows of the energy distribution of theexcimer laser in its x and y coordinates respectively.

FIG. 2 is an illustration of an apparatus used to practice the presentinvention where the laser beam is scanned across the surface of thetarget contact lens blank.

FIG. 3 is a schematic representation of an optical target which definescertain dimensions critical to the relationship of the scanning modalityto ablation profile.

FIG. 3a is a schematic representation of an optical target definingdimensions and parameters used in defining the effect of ablation onradius of curvature of the target surface.

FIG. 4 shows the surface of a contact lens with a lathed surface.

FIGS. 5 and 6 show the interferogram of a toric contact lens made by theclaimed process.

FIG. 7 shows an interferogram of a commercially available toric contactlens.

FIG. 8 shows the interferogram of a lens whose spherical power has beenchanged by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a new method of modifying opticalsurfaces to produce changes in their spherical or cylindrical refractivepower. This new method employs high energy radiation to ablate materialfrom a contact lens blank in a controlled fashion in order to produce adesired contact lens configuration. In particular, the method takesadvantage of the relatively fixed pulse beam intensity of an excimerlaser beam to sweep across the domain of an optical surface. Bycontrolling the rate at which a beam is swept across the target along agiven axis, the degree of ablation along that axis at any given pointcan be controlled.

A beam from an excimer laser is roughly rectangular in cross-section andhas a roughly uniform radiation intensity across one axis of symmetry.The beam intensity is also not absolutely uniform, however, its profilecan be measured and accounted for in the process. The main aspect of thebeam which is substantially fixed is the pulse intensity profile. Thischaracteristic allows, according to invention claimed and described, tocontrol the photoablation of an optical surface.

FIG. 1 shows a typical cross-section of an excimer laser along with theintensity distributions of the laser along its x and y coordinates. Ascan be appreciated, the intensity across the x axis of the beam issubstantially uniform everywhere except at the edges. The inventiondescribes a method for truncating the beam edges and thus provides abeam to the target of uniform intensity.

Controlled ablation of the complete lens surface is accomplished byscanning the beam along its y axis across the optical surface beingaltered. When this scanning is done at a fixed velocity, the effect issimply to broaden the y axis of the beam profile as experienced by thetarget optical surface.

It should be noted that excimer lasers operate in short pulses of about20 nanoseconds and has a total pulse energy very consistent from pulseto pulse. The ablation process contemplated here requires a multitude ofpulses in order to accomplish the desired objective. During the ablationprocess, which typically lasts from 0.1 to 30 seconds, the excimer laserbeam domain can be scanned across the target optical surface in afashion to effect a linear or nonlinear, smooth and continuous ablationprofile along one of the target's axes.

Scanning the excimer laser beam can be accomplished by at least twomeans. FIG. 2 shows an approach where the beam (1) is reflected 90° by a45 degree fixed reflecting mirror (2). The beam is then reflected 90degrees by a scannable mirror (3), which may be a total reflectancemirror or which can be a partial transmittance mirror. In the event thateither mirror is a partial transmittance, mirror means for monitoringthe beam profile of the excimer laser can be provided. Means for movingthe scanning mirror (4) parallel to the initial beam path are providedwhich allows the mirror to shift the beam from its incident path andscan the beam domain across the target optical surface.

Another method of practicing the invention involves the use of a mirrorwhose angle of incidence to the beam can be varied in order to sweep thebeam domain across the target optical surface. In place of the fixedangle mirror (3) in the apparatus shown in FIG. 2 the mirror would beallowed to pivot in the plane of the axis being swept by the laserthrough the target. No other means, such as the linear servo-mechanism(4) found in the diagram would be necessary to effect the beam scanning,although a combination of linearly scanning the mirror in conjunctionwith pivoting the lens could be used.

Assuming the laser intensity and pulse rate of the laser to besubstantially consistent, the degree of photoablation will be related tothe velocity of the mirror (2) as it moves along axis of z, and theablation would be uniform across the target optical surface in the eventthe mirror velocity was constant.

Since photoablation as a function of y is related to the instantaneousvelocity of the beam along the y axis (or the velocity of the mirroralong the z axis) one can control the degree of photoablation bycontrolling the velocity profile of the mirror as it sweeps the beamacross the target domain.

FIG. 2 also shows the apparatus as having a lens (5) which reduces thebeam and thus increases the intensity of the beam as it is incident onthe target (6). It should be understood that this element is optional inthe practice of the invention, the beam could be used in the presentinvention without reducing its cross-sectional dimensions.

The mathematical expression of the relation of ablation to the scanningof the pulsed beam is described as:

    T=K·H·N/ V                               (1)

where

T is the thickness of material removed;

K is a constant;

H is the repetition rate of the laser (pulse rate);

N is the number of scans;

V is the instantaneous velocity of the beam.

In the event that a continuous laser is employed the relationship couldbe expressed as

    T=K.sup.1 N/V                                              (2)

where K1 is a constant. Both K and K1 are constants for the ablation ofa given material in a specific environment. Thus, these constants aredependent upon target material composition, atmospheric conditionsambient to the target, wavelength of the beam, and beam intensity.

Where the scanning rate is non-uniform, ablation along the target axiswill be defined as

    delta T=K·H·N(V2-V1)/(V1·V2)    (3)

where V1 and V2 are the velocities of the beam on the target at pointsP1 and P2, and delta T is the difference in material removal between thetwo points(see FIG. 3).

In order to change the radius in the vertical plane by delta r, theremoval delta T must correspond to the difference in saggital delta Sbetween the original radius and the new radius at all distances for allpoints in a vertical plane (see FIG. 3a). Since

    delta S =delta R·Y.sub.o.sup.2 /2R.sup.2          (4)

where Y is the distance from the center of the lens to a point P in thevertical plane, hence, assuming P1 to be the initial position at thecenter of the lens

    delta R=(2K·H·R.sup.2)/V.sub.0           (5)

The change in cylinder power is then

    K2·H·N

where K2 is a constant independent of the target radius. This allows oneto ablate a lens with a radius of curvature in x axis which is differentfrom the radius of curvature in the y axis, hence a toric lens. Itshould also be noted that the change in cylinder power does not dependupon the initial curvature of the target. For instance a scan that wouldinduce a change in power of 0.25 diopters in a spherical lens willinduce the same degree of change in curvature in the optical targetsurface irregardless of the initial target's radius of curvature.

The ablation profile is, of course, controlled by the product of thebeam intensity profile function, which includes a pulse rate function,with the scanning velocity profile function. In the case where the beampule rate is constant velocity will be related to ablation as

    V+A/Y.sub.o.sup.2

where V is the instantaneous velocity of the beam as it is scanned, A isan ablation constant, and Y_(o) is the distance of the beam from theaxis of symmetry of the induced cylinder in the target, where suchablation causes a cylindrical component to be induced into the targetsurface.

To accomplish combinations of ablation profiles the velocity functionsneed to be combined so that the resultant profile, Vr, at each point is

    1/Vr=(1/V1+1/V2)

It is also clear that some degree of ablation must be accounted for atevery point in the scan given a fixed pulse rate since the beam cannotbe scanned at infinite speed. Thus all ablation profiles will have aconstant maximum and minimum ablation component built into them.

The desired ablation profile required to induce a change in cylinderpower can also be accomplished by controlling the pulse rate of thelaser. Accomplishing a given profile of ablation is dependent uponcontrolling the product of scan speed as a function of time (V) and thepulse rate of the laser as a function of time.

Alternately, the ablation profile can be controlled with constant sweepvelocity by controlling the pulse rate of the laser. It should beobvious that the repetition rate H could be varied instead of or as wellas the scan velocity for different points on the surface. The amount ofmaterial removed at each point is proportional to the repetition rate asshown in Equation 1 . If the velocity is kept uniform, then thevariation in repetition rate required to produce any given profile canbe readily derived in exactly the same way as for the velocity profileusing Equation 1 at each point.

If several types of profile modification are to be accomplished with auniform velocity, the repetition rate distributions for each individualprofile need to be added together at each point and the resultingrepetition rate distribution will produce the composite profile.

If the velocity is varied as well as the repetition rate then the ratioof H to V must be used to ensure the etch depth at each point agreeswith Equation 1. If multiple profile types are to be added together,then either the repetition rate H or the velocity V must be made thesame for each profile distribution for each point. This is a trivialtransformation using Equation 1. It should also be obvious that theadditive process can still be used where both the repetition rate andvelocity change at all points in the distribution because each point canbe treated separately. The functions generated for repetition rate andvelocity should, however, be continuous over the area to be etched.

The means used to control the movement of the scanning mirror can beprovided by a servo-mechanism driven by a stepper motor controlledthrough digital electronic means. Thus, the velocity profile can becontrolled via a computer program and can be of any desired form.

Another way of practicing this invention requires the target opticalsurface to be moved in a controlled fashion along axis x through thelaser beam domain to effect the controlled ablation. This varies fromthe approach shown in FIG. 2 in that it allows one to move the targetrather than the beam.

Apart from the specific apparatus configuration, the fixed targetconfiguration is determined by the initial target configuration and thedegree of ablation as a function of x and y across the surface of thetarget. For instance, it is possible to form a lens from a contact lensbutton (essentially a disk of contact lens material). In order to formdisk into a finished lens, a substantial portion of material must beablated which will require a substantial time to obtain. On the otherhand, the contact lens blank can be substantially in the form of aspherical contact lens. Thus, if one were making a toric lens withrelative low toricity, the amount of time required to make the lenswould be minimal since only a small amount of ablation along 1 axiswould be required to convert the spherical powered lens to a toric lens.Other surfaces which could be altered include corneas, intraocularlenses, spectacles and other optical elements.

One of the significant advantages of the present method vis-a-vis thestate of the art methods of forming toric powered contact lenses is thatthe present method is better able to produce toric lenses withrelatively low degrees of toricity (in particular, this is where theradius of curvature along the x axis, R curvature _(x), is close to theradius of curvature along the y axis, R curvature _(y). Anotheradvantage of the system is that it is able to produce very accurate highcylinder toric lenses as well. In other words, the two radii beingproduced in the toric lens can be specified to a degree of precision fargreater than is available through state of the art lathing techniques.

The present method can be used to form both front surface toric lensesand back surface lenses, as one skilled in the art should appreciate.The method can also be used to produce high quality spherical lenses andlenses with bifocal optics and other configurations. For instance,bifocal lenses could be produced by masking a domain of the target andinducing spherical power change over the unmasked domain, thus producinga lens with two domains of different powers.

The present method can be used on any plastic contact lens material aslong as the radiation source used has sufficient energy and the properwavelength to induce photo- ablation. Specific materials include thenonhydrated forms of poly 2-hydroxyethylmethacrylate (pHEMA), polyN-vinyl-2-pyrrolidone (pNVP), polymethylmethacrylate(pMMA), andcopolymers of the above as well as other contact lens materials known tothose skilled in the art. Gas permeable materials may be used as well assilicone based contact lens materials, especially fluorosilicone basedmaterials.

The choice of radiation sources used on any specific type of lensmaterial will be subject to a number of factors; wavelength ofradiation, the threshold intensity required to cause photoablationmechanics to predominate over their usual degradation modalities whichwill depend to some degree upon wavelength and material type), andambient atmosphere conditions (some ablative modalities are optimized bythe presence of reactive gases, others require "inert" atmospheres).

It has also been found that the process of ablating plastic biomedicalmaterials often creates differential stress across the surface of thematerial which has been ablated. Surprisingly, this affect can beameliorated by uniformly ablating the whole front surface of the target.By removing this uniform thickness of material, the underlying materialis rendered homogeneous at its surface.

The following examples illustrate some of the applications of thepresent invention. The examples do not exhaust all of the possibilitiesof the present invention to shape optical target surfaces. No examplesare given of corneal shaping although the same procedures used to shapecontact lenses could generally be used to shape the optical surface ofthe eye. In such cases, the cornea would be considered to be a targetoptical surface.

EXAMPLES Example 1

An unhydrated soft contact lens blank with a spherical posterior surfacewas aligned in the apparatus shown in FIG. 2 as the optical targetsurface. This lens was made from Polymacon® material, a material widelyused to make soft contact lenses. The posterior surface of the lens wasscanned by the excimer beam as the scanner was scanned along the z axisin a nonlinear fashion that took more material from the edges of thelens than from the center along one axis. The lens was shown prior tobeing scanned by the excimer laser to be very close to a perfect sphereby interferometeric means. The interferogram of the lens prior to laserablation is shown in FIG. 4. The Figure shows that the whole posteriorsurface of the lens is within a few interference fringes of beingspherical which translates to a deviation across the face of the lens ofless than 1 micron. After being scanned with the laser, per the methodof the invention, the lens is toric. FIGS. 5 and 6 show interferogramsalong the two axis of the posterior lens surface. As is clearly shownalong each of the axis, the lens is within several interference fringesof having a given radii along that axis and some other radii along theother axis. This indicates that the two radii of the toric lens wereprecisely what had been anticipated. In this case the radii of the twoaxii were intended to be 6.996 and 7.115 mm and the actual observedradii were 6.991 and 7.108 mm. For comparative purposes, theinterferometer of a commercially available toric lens is shown in FIG.7. Here at least 20 interference fringes can be seen across theposterior surface of the lens.

Example 2

A lens blank with a spherical posterior surface was mounted in theapparatus shown in FIG. 2. The lens was then scanned along an axisaccording to a function which would change the cylinder power of thelens along that axis. The lens was rotated 90 degrees and the lens wasscanned again using the same scan function as employed on the firstscanning sweep. The lens was then subjected to interferometeric analysiswhich showed that the spherical power of the lens was changed and thatthe lens still possessed an almost perfect posterior surface, albeitwith a different power than the original blank. The interferogram of theablated lens is shown in FIG. 8. As can be seen, there are fewinterference fringes across the whole posterior optical surface of thelens.

The initial radius and final radius of a series of 9 lenses whosespherical radius was altered in this way as shown in Table 1 as well asthe corresponding calculated power change in the finished hydrated lensin diopters.

                  TABLE 1                                                         ______________________________________                                        Initial Radius  Final Radius                                                                             Power Change                                       (mm)            (mm)       (mm)                                               ______________________________________                                               7.496        7.581      0.58                                                  7.493        7.569      0.52                                                  7.499        7.586      0.64                                                  7.500        7.587      0.59                                                  7.493        7.569      0.52                                                  7.502        7.586      0.57                                                  7.502        7.586      0.57                                                  7.493        7.583      0.61                                                  7.497        7.589      0.63                                                  7.501        7.593      0.63                                           MEAN   7.496        7.583      .sup.   0.58 +/-                                                              0.06                                           ______________________________________                                    

Example 3

A lens blank with a spherical posterior surface was mounted in theapparatus shown in FIG. 2. The lens was scanned twice along one axis toinduce a cylinder then turned through a 90 degree angle and scanned oncewith the same parameters. These cylindrical components add to produce asphere change in power and a resultant cylinder change in power. Theinitial radius of the surface was 7.470 mm. After both sets of scans,the surface had two radii in perpendicular meridians of 7.382 and 7.292,indicating a 0.088 mm change in spherical radius with an additional0.090 mm cylinder.

What is claimed is:
 1. A method for producing toric powered contactlenses which comprises the steps of:providing a contact lens blank to anapparatus capable of indexing the position of said lens to the beam pathof an excimer laser capable of photoablating the material of said lensblank, scanning the domain of the excimer laser across the contact lensblank along at least one axis and controlling the product of theintensity of said beam with time in order to control the amount ofablation of said contact lens blank along at least one axis of saidcontact lens blank.
 2. A method for treating the surface of a contactlens which removes the stress induced in said contact lens fromselective and nonuniform ablation of material across the surface of thelens, said method comprising the step of uniformly ablating the wholesurface of said lens by photoablative means.
 3. A method for shaping thesurface of an optical target which comprises the step of controllablysweeping the domain of said optical target surface along an axisaccording to a predetermined sweep velocity profile with a high energybeam in such a fashion to selectively ablate material in a predeterminedfashion along said axis.
 4. The method of claim 3 wherein the sweepvelocity profile is defined by the general formula

    V=A/Y.sub.o.sup.2

where V is the instantaneous velocity of the beam as it is scanned, A isan ablation constant, and Y_(o) is the distance of the beam from theaxis of symmetry of the induced cylinder in the target, where suchablation causes a cylindrical component to be induced into the targetsurface.
 5. A method for changing the spherical curvature of radius of atarget optical surface which comprises the steps of: (a) controllablysweeping the domain of said optical target surface along an axis Xaccording to a predetermined sweep velocity profile with a high energybeam in such a fashion to selectively ablate material in a predeterminedfashion along said axis and (b) repeating step a along an axisperpendicular to said axis X used in step (a).
 6. The method of claim 3wherein said sweep velocity profile is chosen so as to add prism fromsaid target optical surface.
 7. The method of claim 3 wherein said sweepvelocity profile is defined so as to remove prism from said targetoptical surface.
 8. The method of claim 3 wherein said optical targetsurface is a spherical contact lens blank and said predetermined sweepvelocity profile produces toric contact lenses.
 9. The method of claim 3wherein said optical target surface is an intraocular lens.
 10. Themethod of claim 3 wherein said optical target surface is a cornea.